Funny, I never knew that. It does not even need to contain a path to perl, all it needs is an occurance of the string "perl".
If a.pl e.g. starts with "#!end lines properly! -d", then "perl a.pl" starts the debugger.
I hope I find a way to turn this knowledge into a beer at the upcoming YAPC::Europe (even though I think this behavour does not really makes sense).